Phase control in electrical transmission



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E. I. GREEN ET AL PHASE CONTROL IN ELECTRICAL TRANSMISSION OriginalFi-led Jan. 6. 1940 10 Sheets-Sheet z E'ZGREEN INVENTORS: MONK Q Qk MS A7' TOR/V15 V 10 Shee t's-Shgeet 9 $3 wwkom mm @E I Efl. GREEN ET ALPHASE CONTROL IN ELECTRICAL TRANSMISSION Original Filed Jan. 6. 1940 u"IL" April 4, 1944.

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ATTORNEY Patented Apr. 4, 1944 PHASE CONTROL IN ELECTRICAL TRANSDHSSIONMillburn, N. 3., and Newton Monk, Darien, Conn., assignors to AmericanTelephone and Telegraph Company, a corpora- Estill Ibbotson Green,

tion of New York Original application January 6, 1940, Serial No.312,792, now Patent No. 2,284,612, dated May 26, 1942. Divided and thisapplication March I, 1942, Serial No. 433,712

20 Claims.

This application is a division of our application Serial No. 312,792,filed January 6, 1940, allowed December 1, 1941, for Phase control inelectrical transmission.

This invention relates to methods and means for compensating for phasechanges of a transmission line and to their application for any desireduse, examples being for automatically controlling voltage regulation andload distribution of power systems and,to phase synchronization ofbroadcast transmitters.

An object of this invention is to maintain constant phase shift in atransmission circuit.

Another object is the continuous and accurate measurement of phasechange between two widely separated currents or voltages.

Another object is to maintain a constant phase relationship between thecurrents or the voltages at two points separated by an appreciablelength of line.

Another object is to maintain identical phase of the carrier frequencyof a plurality of common frequency broadcast stations or identical phaserelationship of the currents or voltages in different systems.

Another object is to provide a simple and continuously operating systemfor controlling in accordance with phase relationship the voltageregulation and load distribution in power systems.

Another object is to provide for relative phase control of certainoperating factors as between different parts of a power system or asbetween different power systems.

A further object is to provide means for automatically carrying out theabove-named objects.

Frequency synchronization in electrical systems is a usual requirement,while phase synchronizaticn is not always necessary. This inventionembraces the general problem of maintaining constant phase shift for asubstantial length or line and is directed to methods and means forautomatic compensation of variations in phase change of transmissionlines, thereby securing constant phase shift.

In applying this invention to the control of other systems a phasecompensated pilot line .or the equivalent is generally employed indetermining the difference in phase of a voltage delivered to the phasecompensated circuit at one point with that of a local source at anotherpoint by connecting a phase angle meter or other suitable preciablelength, for instance, such as'might be used as a pilot line,does notremain constant with time but varies with changing temperature andweather conditions. For cable circuits the variation in phase shift isdue to temperature changes which affect principally the resistance andto a lesser extent other circuit constants. For open-wire lines thechanges in phase result largely from changes in leakage conductance ofthe insulators, although temperature effects are also important.

Compensation for variations in phase change of a line as disclosedherein may be brought about by three different methods:

(1) Direct current method;

(2) Alternating current method; and

(3) Carrier current method.

The direct current method is primarily applicable to cable circuits.Further, the arrangement is best adapted to cable circuits operating atrelatively low frequencies. This method comprises maintaining constantphase shift for substantial length of line circuits by taking advantageof the relation of the phase shift to resistance. This relationship maybe mathematically expressed. For cable circuits at low frequencies, thepropagation constant is given by the following approximate formula:

Since the capacitance C changes very little with temperature, it will beseen that the change in the phase shift 5 results largely from thechange in resistance.

device between the local source and the pilot line reaching this point.

The phase shift characteristic of a line of ap- In the first arrangementhere shown, the variation in phase shift of the line circuit isautomatically compensated through the agency of a self-balancingWheatstone bridge circuit. This arrangement employs direct current toeffect the phase compensation of the line. The results obtainable bythis first method are approximate while the latter two methods abovementioned are exact.

When it is desired to obtain constant phase shift on several circuitswhich are exposed to the same conditions, such as several circuits inthe same cable, the arrangement applicable to one circuit may beslightly modified and one of the circuits which may be termed a pilotcircuit may be used to correct the phase variations of all of thecircuits. This avoids duplication of the compensating apparatus for eachcircuit.

The alternating current method is not limited to making approximatephase change correction on the basis of resistance variation only. Itemploys an alternating current in maintaining constant phase shift forsubstantial lengths of line circuits by an arangement which obtainssubstantially perfect correction of phase change and i applicable aliketo cable or to open-wire lines.

The carrier current method comprises maintaining constant phase shiftfor substantial lengths of line circuits by an arrangement which alsoobtains substantially perfect correction of phase change for whatevercause and is applicable alike to cable and to open-wire lines andparticularly to high frequency operation. This arrangement employs acarrier current which is modulated by a low frequency current at thetransmitting station for use in effecting the phase shift compensationof a line.

With any of the three methods the line can, with suitable arrangements,be used simultaneously for other purposes.

A description of a number of embodiments chosen for illustrating thisinvention in which reference is made in each case to the accompanyingdrawings follows:

Figure 1 shows an arrangement employing direct current for the automaticcompensation of phase change variations of a line in which advantage istaken of the relation of the change in phase to the change inresistance.

Fig. 2 shows more in detail the automatically operated adjustingmechanism of Fig. 1 and of other figures.

Fig. 3 shows a possible type 01' circuit arrangement for the phaseadjuster of Fig. 1 and of other figures.

Fig. 4 shows a typical circuit arrangement of the phase angle meter ofFig. 1 and of other figures.

Fig. 5 shows arrangements employing alternating current for theautomatic compensation of phase change variations of the line in which asingle pilot current of the same frequency is transmitted between twopoints in opposite directions over the same line using hybrid coils.This figure also shows how the arrangement may be employed to obtaincomparisons of the phase relationship of voltages at the two diflerentpoints.

Fig. 6 shows a hybrid coil type of line repeater.

Fig. '7 shows a possible arrangement for automatically controlling thevoltage or load at a power station in accordance with the phaseindications transmitted over a compensated line.

Fig. 8 shows an arrangement employing alternating current for theautomatic compensation of phase change variations of a line in which asingle pilot current of the same frequency is transmitted between twopoints in opposite directions over two different lines. This figure alsoshows how the arrangement may be employed to obtain comparisons of thephase relation of voltages at the two different points.

Fig. 9 shows an arrangement employing alternating current for theautomatic compensation of phase change variations of a line usingfilters in which pilot currents of two different frequencies aretransmitted between two points in opposite directions over the sameline. This figure also S h0Ws how the arrangement may be employed toobtain comparison of the phase relationship of voltages at the twodifierent points.

Fig. 10 shows a filter type line repeater.

Fig. 11 shows an arrangement employing alternating current forautomatically compensating a two-wire line for phase change variationsbetween two points arranged for making successive phase comparisons ofthe voltages of all three phases of a power system or systems at the twopoints between which the compensated pilot line extends. Synchronousswitching mechanism is used to connect one phase after another.

Fig. 12 shows an arrangement employing alternating current forautomatically compensating a. three-wire pilot line for phase changevariations between two points arranged for simultaneous phasecomparisons of the voltages of all three phases of a power system orsystems at the two points between which the three-wire compensated pilotline extends. Suitable coupling means are employed to connect the pilotline with the power lines to simultaneously compare all three phases ofthe power system or systems.

Fig. 13 shows an arrangement employing carrier current for automaticcompensation of phase change variations of a line in which a pilotcurrent consisting of a carrier current modulated by a-low frequencycurrent is transmitted between two points in opposite directions overthe line and illustrates how the arrangement can be employed for makingcomparisons of the phase relationship of independent voltages at twodifl'erent points.

Fig. 14 shows an arrangement employing carrier current for automaticcompensation of phase change variations of a line in which the pilotcurrent in one direction consists of the carrier current modulated by alow frequency current and in the opposite direction of a harmonicfrequency of the carrier current, thus using diflerent pilot freqenciesfor transmission in opposite directions over the line and alsoillustrates how this arrangement can be employed for making comparisonof the phase relationship of independent voltages at two differentpoints.

Fig. 15 shows an arrangement employing carrier current for automaticallycompensating a pilot line for phase change variations between two pointsof a power system in which the carrier current modulated by the powercurrent is transmitted over the automatically compensated pilot line andin which synchronous switching mechanism is used to connect in one phaseafter another of the power current for making phase comparison of thepower current or voltages at two different points.

Fig. 16 shows an arrangement employing carrier current for automaticallycompensating a pilot line for phase change variations between two pointin which three different frequency carrier currents modulated by thepower current of the diflerent respective phases of the power system aretransmitted over the automatically compensated pilot line forsimultaneously making phase comparisons of the power currents orvoltages.

Fig. 17 shows an arrangement employing car-- rier current forautomatically compensating a three-phase pilot line for phase changevariations between two points arranged formaking phase comparison of thecurrents or voltages of all three phases of a-power system at twodifferent points in which three different carrier currents are modulatedby the power current of the diflerent respective phases of the powersystem and are transmitted over the respective phases of theautomatically compensated three-phase pilot line for simultaneouslymaking phase comparison of the power currents or voltages.

Fig. 18 shows an arrangement employing carrier current for automaticallycompensating one phase of a three-phase power line for phase changevariations between points in which the compensation is achieved bytransmitting a modulated carrier current in opposite directions over thecompensated one phase of the power line wire and arrangements at onepoint for making comparison of phase relationship of the currents orvoltages at the two difierent points.

Fig. 19 shows an arrangement employing carrier current for automaticallycompensating one phase of a three-phase power line for phase.

phase of a three-phase power line for phase change variations betweentwo points in which compensation is achieved by transmitting a modulatedcarrier current in opposite directions over the compensated one phase ofthe power line wires between the two points and in which synchronousswitching mechanism is used to connect in one phase after another of thepower circuit for making phase comparison of the power cur rent orvoltages of the three-phase system at the two different points.

Fig. 21 shows an arrangement employing carrier current for automaticallycompensating all three phases of a three-phase power line for phasechange variations between two points in which compensation is achievedby transmitting three different suitably modulated carrier currents inopposite directions over the compensated power line wires of each phasebetween the two points and arrangements at one point for makingcomparison of phase relationship of the current or voltages in thedifferent phases of the power circuit at the two different points.

Fig. 22 shows an arrangement employing carrier current for automaticallycompensating one phase of a three-phase power line for phase changevariations between two points in which comparison is achieved bytransmitting three different modulated carrier current frequencies inopposite d rections over the compensated one phase of a power linebetween the two points and arrangements at one point for simultaneouslymaking comparison of the phase relationship of the currents or voltagesof the different phases of the three-phase power circuit at the twodifferent points. 7

Fig. 23 shows an arrangement employing carrier current for automaticallycompensating for phase change variations of pilot lines interconnectingradio broadcast stations forachieving I00. In transmission over the linethe current or voltage undergoes a certain phase shift and by means of aphase adjuster I50 the phase shift may beneutralized completely orpartially. The pilot line I00 itself comprises one arm of the bridgeI30, the resistance I3I which is made equal to the average resistance ofthe line forms a second arm, while resistances I32 and I33 are the otherarms. Between the line and the resistance I3I is connected a variableresistance I35 for balancing the bridge. Across the opposite points ofthe bridge is connected a polar relay I 36.

When the resistance of the line I00 changes, this relay closes eitherits upper contact I31 or its lower contact I38, depending upon thedirection of change. Automatic adjusting mechanism I then adjusts theresistance I35 until the bridge is once more balanced.

The automatic adjusting mechanism I40, shown more in detail in Fig. 2,is such that its motor Ill rotates in one direction when the uppercontact I31 of'the polar relay is closed and in the other direction whenthe lower contact I38 is closed.

The motor of the adjusting mechanism is me-' chanically geared to thevariable resistance I35 and also to the phase adjuster I so that inaddition to rebalancing the bridge, the adjusting mechanism serves alsoto actuate the phase adjuster to compensate for the change in phasewhich accompanies the change in line resistance. Since only a singlefrequency is involved, the phase adjuster I50 may be quite simple.

The phase adjuster, for example, may be simply a variable condenser orinductance or it may be of a somewhat more complicated type. A possibletype of phase adjuster is shown in Fig. 3. The total range of the phaseadjuster I50 must be as' great as the maximum variation in the phaseshift of the line I00. Two or more sections like that shown in Fig. 3may be used in series in case suflicient phase shift is not obtained ina single section.

In the circuit connecting the line with the bridge, a filter I20 isprovided to keep the alternating current from the source I0 and othersuperfluous frequencies out of the bridge circuit. Condensers I2I andI22 are provided to isolate the receiving apparatus at station B fromthe bridge. Where it is desired to have the phase of the source I0 thesame at the receiving point as that at the point of origin a phasecompensator I10 may be inserted in the line to provide a constant phaseshift opposite to the net phase shift introduced by the line, and thephase adjuster I50. It is possible in most cases. however, to omit thephase compensator phase synchronization of broadcast transmittersemploying common frequency carriers.

Similar reference characters refer to corresponding parts in thedifferent figures of the drawings.

Fig. 1 shows an arrangement for the automatic compensation of phasechange variations of a line in which advantage is taken of the relationof change in phase to change in resistance as expressed in equation (1).In this arrangement,

variations in phase shift of the pilot line I00 are automaticallycompensated through the agency of a self-balancing Wheatstone bridgecircuit. At station A a source of current I0 provides alternatingcurrent for transmission over pilot line I10 and if necessary tocalibrate the receiving apparatus accordingly. In order'to determine theamount of phase shift to be provided by the phase compensator I10, itisnecessary to know the phase shift of the line. This may be obtainedfrom calibrations based on the fundamental constants of the circuit ormeasurements of the phase change may be made by employing one of thevarious methods discussed by Messrs. Nyquist and Brand in an articleentitled The Measurement of Phase Distortion," published in the BellSystem Technical Journal of July 1930, volume IX, commencing on page522. With the exception of the open and short-circuit impedance method,which is applicable only to lines which are electrically short, all ofthe methods discussed by Messrs. Nyquist and Brand require that bothends of the circuit be available at the same location.

This is usually achieved by employing two similar c1. cuits, loopingthem together at the distant end and dividing the measured phase changeby two. Where two similar circuits are not available, the methoddescribed in the copending application of Messrs. E. I. Green and N.Monk, Serial No. 259,494, filed March 3, 1939, now U. S. Patent No.2,214,130 of September 10, 1940, may be employed.

The compensated pilot line provided by the arrangement shown in Fig. 1may be used for indicating the difference in phase between the currentsor voltages of two sources at stations A and B, respectively. At stationA a source I and at station B a source 2I0 separately providealternating current power of the same frequency. The separate sourcesmay be supplying power to one transmission system or to two difierenttransmission systems. In supplying power to the same or differentsystems the load distribution or its division as between the sources maybe controlled in accordance with the phase angle displacement betweenthe respective power currents or voltages. By the arrangement hereshown, the phase angle between the two respective currents or voltagesis indicated and variations in phase change in the pilot line whichmight give rise to false indications are eliminated. A voltage V1generated by the source I0 at station A after being transmitted overline I00 undergoes a certain phase shift and is now designated V'i. Bymeans of the phase adjuster I50 the phase shift may be neutralizedcompletely or partially, as heretofore described, and after suchneutralization of the phase shift has taken place is designated V"1.This voltage V"! of known phase shift with respect to voltage V1generated by source I0 may be utilized for measuring the phasedifference between voltage V1 and V2 generated by the sources I0 and 2I0 at stations A and B, respectively. The means for utilizing thereceived frequency after phase correction is shown in Fig. 1 as a phaseangle meter 200. This meter may be used to indicate the difference inphase between the voltage V"1 delivered by the phase correctingmechanism and the local voltage V2.

The indications of the phase angle meter or of a recorder may be used asa guide for the control of load distribution between two generatingstations whose voltages are being compared. Such control may beaccomplished through manual adjustment of tap changers or other deviceswell known in the power industry. It is also possible, as discussedlater, to utilize the phase relationship between the voltages of the twostations to obtain automatic voltage control.

The phase angle meter 200 may be of any wellknown type such as, forexample, the Tuma phase meter diagrammatically shown in Fig. 4. Thisarrangement consists of a fixed coil 20I within which two similar coils202 and 203 rigidly set at right angles to each other are allowed torotate, freely. The voltage V"1 is passed through the fixed coil 20Iwhile the voltage V2 is passed through both of the rotatable coils 202and 203 in quadrature, this being accomplished by inserting a resistance204 in series with coil 202 and an inductance 205 in series with coil203. If desired the phase angle meter 200 may be a phase angle recordersuch as, for example, one of the type shown in Fig. 4 of the article byForbes and Searing entitled Voltage and Load Control published inElectrical Engineering, June 1934.

When it is desired to obtain constant phase shift on several linecircuits which are exposed to the same condition, for example, severalcircuits of the same cable, it is possible to apply the arrangement ofFig. 1 in a slightly different way. This is also applicable toarrangements subsequently described herein. One line circuit which mightbe termed a pilot circuit would be used in connection with a singlebridge and rebalancing mechanism and this mechanism would serve tocorrect the phase on a number of other circuits.

Fig. 5 shows an arrangement for obtaining substantially perfectcorrection of variations of phase change introduced in a transmissionline. This arrangement is applicable alike to both cable and open-wirelines and to low or high frequencies. Fig. 5 shows an arrangementemploying alternating current for the automatic compensation of phasechange variations of a line in which the alternating current methodabove mentioned is employed. A pilot current of the same frequency istransmitted between two points in opposite directions over the sameline, using hybrid coils, for making comparisons of the phaserelationship of currents or voltages existing at the two differentpoints. Stations A and B at distant points include the sources I0 and2I0, respectively, for generating alternating current of low frequencyand by the arrangement shown the phase angle between the two currents orvoltages is indicated and can be employed for purposes of load control.In this arrangement variations in the phase shift of the line I00 areautomatically compensated through the agency of a balanced rectifier andrelay 30 which causes the operation of adjusting mechanism 40 which inturn controls phase angle ad- J'usters 50 and 60 in accordance with theaction of the balanced rectifier 30. Alternating current from source I0at station A after passing through current limiting resistances II, thephase adjuster 50 and hybrid coil I20 having a balancing network I2I istransmitted over the line I00 to station B where a part of the incomingenergy is taken off through hybrid coil 220 having a balancing network.22I and after being amplified in amplifier 290 which should be designedfor constant phase shift is returned to the line through a conjugatepair of terminals of hybrid coil 220. At station A part of the returnedenergy is withdrawn from the line I00 through hybrid coil I20 and afterpassing through phase adjuster 60 is applied to the common gridconnection of the balanced rectifier 30. Also, alternating current fromthe source I0 after passing through current. limiting resistances I2 andphase compensator I0 is applied to the balanced rectifier 30. Thisrectifier arrangement has the well-known property that when the twoinputs differ in phase by degrees, or more generally by n1r/2 where n isany integer, the outputs of the two rectifier tubes will be equal. Thisis true regardless of the relative amplitudes of the two inputs. 11,however, the 90 degree phase relationship between the inputs is altered,the polar relay 36 will operate in one direction or the other dependingon the direction of the phase difference. The operation of this polarrelay controls the operation of adjusting mechanism 40 which may besimilar to that shown in Fig. 2. The operation of this adjustingmechanism in turn simultaneously adjusts the phase adjusters 50 and 60to restore the condition of balance. Assuming that the changes in phaseshift are the same for either direction of transmission over the line,

the phase shift introduced by the phase adlusters 50 and 60 should bemade equal. It will then be evident that as long as a condition ofbalance holds at station A, the phase of the received currents orvoltages at station B will always have the same relationship to that ofstation A. The total range of adjustment of each of the variable phaseadjusters 50 and 60 must be as great as the maximum variation of thephase shift of the line. The phase compensator may be employed toinitially obtain the desired 90 degree phase relationship be- 30. Thephase angle difference between the alternating currents or voltagesgenerated by sources l0 and 2|0 at the two stations Aand B,respectively, is shown by the phase angle meter 200. Alternating currentfrom the source I0 after being transmitted over the compensated line I00is impressed on one side of the phase angle meter 200 and alternatingcurrent from the source 2I0 is impressed upon the other side of thephase angle meter. High resistance 2 may be inserted in the leads fromthe compensated line at station B to provide thenecessary impedance toprevent losses to the line by the connected terminal apparatus. A phasecompensator 210 may be inserted to provide a phase shift opposite to thephase shift introduced by the line so that the total phase differencebetween the currents or voltages of the sources i0 and 2H! may be readdirectly. However, this phase compensator 210 may be omitted and ifdesired the receiving apparatus calibrated accordingly. In order todetermine theamount of phase shift to be provided by thephasecompensator 210, it is necessary to know the phase shift of the line.This may be obtained from computations based on the fundamentalconstants of the circuit or measurements of the phase change may be madeby employing one of the various methods referred to in describing Fig.1.

Fig. 6 diagrammatically shows the well-known hybrid coil type linerepeater which may be used in the line-circuit I00. One or more of theseor '15 tween the two inputs to the balanced rectifier Kill othersuitable types of repeaters may be used in various arrangements heredescribed.

If it is desired to employ the phase comparisons obtained with thearrangement of Fig. 1 or Fig. 5, as a basis for automaticallycontrolling the load distribution on a system, this can be done by meansof the arrangement shown in Fig. 7 which may be substituted for thephase anglemeter shown in Figs. 1 and 5, the circuit connections beingmade at the point indicated by the dashdot line X-X at station B.

Fig. 7 shows an automatic load control ar-.

rangement. It employs a balanced rectifier phase comparison arrangement230 similar to that shown at station A in Fig. 5 which in this casephase angle control between two intermediate power systems.

Fig. 8 shows a compensation arrangement employing two pilot lines, anoutgoing and a returning pilot line. This shows an arrangement alsoemploying alternating current for the automatic compensation of phasechange variations of a line in which a single pilot current of 'the samefrequency is transmitted between two points in opposite directions overtwo different lines and it also shows how the arrangement may beemployed to obtaincomparisons of the phase relationship of currents orvoltages at two different points. Stations A and B at distant points areprovided with sources I0 and 250, respectively, for generatingalternating current of the same frequency. In this arrangement thevariations in the phase shift of the outgoing line circuit I00 and thereturning line circuit H]! are automatically compensated and heldsubstantially constant through the agency of a balanced rectifier andrelay 30 which causes the operation of an adjusting mechanism 40 whichin turn controls the phase angleadjusters and in accordance with theaction of the balanced rectifier 30. Al-

ternating current from the source In at station 1 A passes throughcurrent limiting resistances l l and phase adjuster 50 for transmissionover the line I00 to station B and back over the return line ill! to thephase adjuster $0 and thence to the balanced rectifier 30 at station A.The balanced rectifier 30 and the adjusting mechanism 10 are similar tothose shown in detail in Figs. 5 and 2, respectively. Current from thesource it at station A is also fed through current limiting resistancesl2 and phase compensator to the balanced rectifier 30. The operation ofthe bal- 1 anced rectifier causes the adjusting mechanism 40 tosimultaneously operate the phase adjusters 50 and 60 so as to restorethe condition of balance and thus automatically compensate the lines 500and NI interconnecting stations A and B. The phase compensator I0 may beemployed to obtain initially the desired degree phase relationshipbetween the two inputs to the balanced rectifier 30. The phaseangle'adjusters 50 and 00 must be of ample range to compensate for bothlines. The action of the phase adjusters and the balanced rectifier issimilar in this arrangement to that described more in detail for Fig. 5.The phase angle difference between the alternating currents generated atthe two stations is shown by phase angle meter 200. Ourrent from sourceit! after being transmitted over compensated line I00 is impressed uponphase angle meter'200 at the same time that current from source 2 I 0 isimpressed upon the phase angle meter. High resistances 2!! in the leadsfrom the compensated line to the terminal apparatus prevent thisapparatus from aliecting transmission over the pilot lines. A phasecompensator 210 may be inserted to provide a phase shift opposite to thephase shift introduced by the pilot line I00.

In the arrangements so far described, the pilot.

currents employed are the same 'freduency for opposite directions oftransmission or in other words, for both directions; of transmission.However, in some instances it may be advantageous to employ differentfrequencies for opposite directions of transmission. Fig. 9 shows an ara pilot current of different frequencies is transmitted between twopoints in opposite directions over the same line and this figure alsoshows how the arrangement may be employed to obtain comparison of phaserelationship of currents or voltages at the two different points. Inthis case a frequency ii is transmitted over the line to the receivingend and a harmonic derived from this frequency, for example 2h, isreturned to the transmitting end where its phase is compared with thatof the same frequency derived locally from the original source. In thiscase the phase adjusters 50 and 60 are not alike and each must correctfor phase change variations at the particular frequency transmittedthrough it. While it would be possible with this arrangement to usehybrid coils to separate the oppositely directed currents, the use ofdifierent frequencies in opposite directions makes it possible to employfilters instead of hybrid coils atthe terminals and repeaters. StationsA and B at distant points are provided with sources I and 2I0,respectively, for generating alternating currents of frequency f; whosephase relationship is to be compared and by means of the arrangementshown the phase angle between the two currents or voltages is indicated.In this arrangement variationof the phase shift of line I00 isautomatically compensated through the agency of the balanced rectifierand relay 30 which cause operation of adjusting mechanism 40 which inturn controls phase angle adjusters 50 and 60 in accordance with theaction of balanced rectifier 30. Alternating current from source I0 atstation A after passing through current limiting resistances II, phaseadjuster 50 and filter 5| transmitting frequency h, the same as thatfrom source I0, is transmitted over the line I 00 to station E where apart of the incoming energy is taken off through filter 25f transmittingfrequency f1 and after being amplified in amplifier 290 is transmittedto harmonic generator 260 from which a harmonic of this frequency isderived, for example 2h, which is transmitted through filter 26I back tothe line I00 and to station A. At'station A the returned energy iswithdrawn from the line I00 through filter 6|, transmitting frequency Znand after passing through phase adjuster B0 is applied to the commongrid connection of balanced rectifier 30. Also current from the sourceI0 is transmitted through current limiting resistances I2 to harmonicgenerator 20 where a harmonic frequency 2j1 is derived which istransmitted by filter 2I passing frequency 2;1 through phase compensator10 to the balanced rectifier 30. The operation oi? balanced rectifier 30controls the operation of adjusting mechanism 40 which causes phaseadjusters 50 and to operate to restore the condition of balance and thusautomatically compensate the line I00 for changes in phase shift. Inthis arrangement a harmonic of the pilot current frequency. is impressedupon both sides of the balanced rectifier instead of a current of theoriginal pilot frequency. Current from source I0 after being transmittedover compensated line I00 is also impressed upon one side of phase anglemeter 200 and current from source 210 is impressed upon theother side ofthe phase angle meter 200, and the phase angle difference between thecurrent or voltages generated at the two stations A and B is shown bythis phase angle meter. High resistances 2| I are in the leadsconnecting the compensated line and the terminal apparatus to limittransmission losses due to the bridged apparatus. A phase compensator270 may be inserted to provide a phase shift opposite to the phase shiftintroduced by the line.

Fig. 10 shows the well-known filter type line repeater, particularlyapplicable for use with the arrangement shown in Fig. 9. One side of therepeater transmits frequencies of f1 and the other side frequencies ofM1 or any other harmonic nfi of the fundamental frequency f1 which maybe selected or used. Such repeaters are well known in the communicationsand signaling art.

Fig. 11 shows an arrangement employing alternating current forautomatically compensating a two-wire line for phasechange variationsbetween two'points arranged for making successive phase comparisons ofthe voltages of all three phases of a power system or systems. At thetwo points between which the compensated power line extends, synchronousswitching mechanism is used to connect in one phase after another ateach point. In this arrangement the automatic compensating equipment atstations A and B may be similar respectively to that shown in Figs. 5 or9 for compensating the pilot line I00. Connection with the power linesor systems at the two stations is made through synchronous switchesarranged for making connections in succession with the different phasesof the power systems. As here illustrated the terminal apparatus at eachstation is connected with the power system circuits through suitableprotective condensers and resistances to keep the power currents off thepilot line and its-terminal apparatus. At station A the three-phasepower circuit 5I0 is tapped by three leads through protective condensers52I, 522 and 523 and respective series resistances 525, 526 and 521 ineach lead to connect the different phases A, B and C with thesynchronous switch 3I0 which connects through the automatic phasecompensating equipment 99 with the compensated pilot line I00.leading tostation B. At station B the three-phase power circuit 6I0 is connectedby three leads through protective condensers MI; 822, 623 and respecative series resistances 625, 625 and 621 in each lead to connect thedifferent phases A, B and C of the power circuits through respectivephase angle meters 200A, 2003 and 2000. These are in turn connected withthe compensated line I00 through the automatic phase compensatingequipment 299 and the synchronous switch H0. The synchronous switches atthe two stations A and B may be of any well-known type and operated insynchronism by any suitable means, not shown inuthe drawings, to connectthe different corresponding phases of the power systems at the twostations for making phase angle comparisons. Current from each phase ofthe power system at station A, after being transmitted over thecompensated line I00, successively impressed upon the phaseangle meters200A, 2003 and 200C of the respective phases A, B and C for making thephase comparisons with the currents or voltages of th power system atstation B which is also successively impressed upon the phase anglemeters. In this arrangement the automatically compensated pilot line I00and the terminal apparatus at each station makes it possible to comparethe phase angle difference of the currents or voltages in the respectivephases of the two power systems at any time by operating the synchronousswitches at the two stations manually or they may be continuouslyoperated automatically at such a speed as to permit continuous readingson the phase angle meters.

In the arrangements so far discussed, two-wire pilot lines have beenemployed without recourse to multi-channel carrier current operation andphase comparisons were thus limited to comparing single phase circuitsor sources at a time so that when polyphase systems were, involved thecomparisons were necessarily made in succession as'shown in Fig. 11.

Fig. 12 shows an arrangement for making phase comparisons of each of thethree phases of a power system or systems simultaneously. This figureshows an arrangement employing alternating current for automaticallycompensating a three-wire pilot line for phase change variations betweentwo points arranged for simultaneous phase comparisons of the currentsor voltages of all three phases of a power system or systems at the twopoints between which the three-wire compensated pilot line extends.Suitable coupling means are employed at the points to connect thethree-wire pilot line with the power lines for simultaneously makingphase relationship comparisons between all three phases of the powersystem or systems. One phase of the three-wire pilot automaticallycontrols the compensation of all of the three phases of the pilot lineas, where the circuits of the three-wire pilot line are close together,one circuit may adequately serve to govern the compensation of allcircuits as heretofore stated. The automatic phase compensatingequipment here shown is of the filter type. -As it is not practicable toconnect the terminal apparatus directly to the power line some form ofcoupling must be employed as pointed out in describing Fig. 11. Atstation A leads from the power circuit 5I0 connect through condensersRI, 222

and 223 to the three-phase filter 530, transmitting frequency hfwhich inturn is connected to the three-phase pilot line I05. While current fromthe power line is'transmited over all three phases of the three-phasepilot line I05, the compensat ing apparatus is controlled by one phaseof the pilot line as stated above. Alternating current from one phase ofthe power line, after transmission over one phase of the pilot line I05and through three-phase filter 630, transmitting frequency h, is takenoff at station B through modulator 264 and by means of oscillator 265 isstepped up to a different frequency f2 and returned through filter 263to the one phase of the pilot line and then over it to station A. Thereturn frequency 1: is passed by filter 63 to demodulator 65 from whichcurrent of frequency [1 is transmitted to the balanced rectifier 30,while current of frequency j1 from the power line at station A is passedthrough filter 22 which transmits frequency f1 to phase compensator l0and to the balanced rectifier 30. The balanced rectifier controls theoperation of the adjusting mechanism fl0which in turn drives the phaseadjuster 80 which operates on all three phases of the pilot line i435,thus automatically compensating each circuit or the pilot line. Thepilot line and phase adjuster should have a phase change which is linearwith frequency since the pilot line transmits current in oppositedirections of different frequencies. High resistances H2 and H3 atstation A, and H4 and H5, 'and also H6 and H7 at station B serve tobalance the two phases of the pilot line which are not connected to theautomatic compensating equipment. At station 3 phase angle meters 200A,2003 and 2006 are connected through phase compensator 610 with thedifferent phases of the compensated three-phase pilot lines I05 andthrough condensers 62!, 622' and 623 to the corresponding phases of thepower system 6I0. The phase angle meters, therefore, show at all timesthe phase relationship of the currents or voltages in the difierentphases of the circuits of the power systems 5I0 and (H0. The returnfrequency f2 over one of the phases of the pilot lines I05 may have aharmonic relationship to the frequency of the power line current or itmay be of any other suitable frequency. Subsequently an arrangement willbe described herein in which the power lines are used for transmittingthe compensating currents and for operating the compensating apparatusas well as transmitting the power current.

It may happen in some instances that the line is not adaptable totransmitting low frequencies or it may be desirable for some otherreason to employ high frequency transmission over the pilot line.-

Fig. 13 shows an arrangement employing carrier current for automaticcompensation of phase change variations of a line in which a pilotcurrent consisting of a carrier current modulated by a low frequencycurrent is transmitted between two points in opposite directions overthe line and illustrates how the arrangement can be em-. ployed formaking comparisons of the phase relationship of independent currents orvoltages at two different points. The arrangement for achieving theautomatic compensation of phase change variations of the pilot line isshown as similar to Fig. 5 but modified to employ a carrier current. Atstation A alternating current of frequency f1 from source I0 modulatesin modulator SI 2. carrier current generated by oscillator 90 and theupper (or lower) side-band frequency is transmitted through filter 92,phase adjuster 50,

hybrid coil I20 and high-pass filter H8 to thepilot line I00. As thepilot line I00 here shown isa section of a telephone circuit, theabove-mentioned currents in reaching the pilot line pass throughhigh-pass filter II8 which prevents the telephone signals on the pilotline reaching the te minal apparatus. The telephone circuit is c tinuedthrough low-pass filter H9. tibn B the pilot line connects with theterminal apparatus through high-pass filter 2I8 and with the telephonecircuits through low-pass filter 2I9. At station B a part of the highfrequency current is amplified and returned by means of hybrid coil 220over the pilot line I00 to station A where it passes through phaseadjuster B0, demodulator and filter 65, which passes only currents ofthe same frequency f1 as that of source I0 to the bal anced rectifier30. The low frequency source of current I0 transmits frequency f1through the phase compensator I0 to the other side of the balancedrectifier 30. The balanced rectifier controls the adjusting mechanism 40which in turn operates the phase adjusters 50 and 60 to automaticallycompensate the pilot lines I00 ina manner similar to that explained morein detail At staphase relationship is to be compared with that of thecurrent from source l0. Thus the phase angle motor 200 indicates thephase relationship between these two sources. Thi figure and subsequentfigures show various arrangements employing carrier current forcompensating a pilot line and various applications of such compensated111188 to indicate phase relationship and to compare the phaserelationship of the currents or voltages of power systems.

Whife the carrier current arrangement shown in Fig. 13 as in subsequentfigures shows two phase adjusters at station A, it is possible to useonly one adjuster in certain of the carrier current arrangements. Fig.12 shows a Single phase adjuster and the arrangement in that figure maybe considered a partial carrier current system in that the returnedcurrent modulates a carrier frequency. In this case the phase adjustermust adjust for currents in both directions of trans- 'mission. Sincethe phase change of a line will be practically linear with frequency. asingle phase adjuster incorporating corrective phase change for both f1equencies may be employed.

Fig. 14 shows an arrangement employing carrier current for automaticcompensation of phase change variations of a line in which the pilotcur.- rent in one direction consists of the carrier current modulated bya low frequency current and in the opposite direction of the harmonicfrequency of the carrier current thus using carrier currents ofdifierent frequencies for the trans mission in opposite directions overthe line and also illustrates how this arrangement can be emplayed formaking comparisons of phase relationships of independent currents orvoltages at two different points. Fig. 14 is similar to Fig. 13 exceptthat the returned current is a harmonic of the transmitted current andfilters instead of hybrid coils are employed for separating the twocurrents. In this arrangement a frequency f1 supplied by sourcemodulates in modulator 9| a carrier frequency f2 supplied by oscillator90 and the upper (or lower) side-band is transmitted by filters '92, 93and H8 to the .pilot line I00. The current upon reaching station B istransmitted by filters H8 and 25l and amplifier 290 to harmonicgenerator 260. The returned harmonic frequency may be 2(f1+f2). It istransmitted back to station A through filters 26L H8, H8 and 6| to phaseadjuster 60 and to one side of the balanced rectifier 30. At station Apart of the upper side-band current from modulator 9| and filter 92 isimpressed upon harmonic generator 20 and the harmonic component having afre-- quency 2(,f1+fz), the same as that returned over the pilot lineI00, is passed through filter 2| and phase compensator 10 to the otherside of the balanced rectifier 30. The balanced rectifier 30 in turn}controls the operation of the adjusting mechanism 40 which operatesphase adjusters 50 and 60 to automatically maintain compensation of thepilot line I00. transmitted current is also passed through highresistance 2| I, phase compensator 210, demodulator ZSI and filter 265which transmits current having a frequency f1 to the phase angle meter200. A source of current 2|0 having the frequency I1 is connected to theother side of the phase angle meter 200 and the relative phase.displacement of the currents or voltages generated by sources l0 and 240is shown on this phase angle meter at station E.

Fig. shows an arrangement employing car- I rier current forautomatically compensating a figures such as Fig. 13 or Fig. 14, thecompensat- I At station B a part of the pilot line for phase changevariations between two points of a power system in which the carriercurrent modulated by the power current is transmitted over theautomatically compensated pilot line and in which synchronous switchingmechanism is used to connect in one phase after another of the powercircuits for making phase comparisons of the power currents or voltagesat two different points. This figure illustrates the application or useof a compensated pilot line for making phase comparison of poly-phasepower systems and at the same time using the pilot circuit for anotherpurpose such as a telephone circuit. The automatic phase compensatingequipment 99 and 299 at stations A and B, respectively, may be the sameas shown in other drawing ing apparatus being shown within therectangular dot-dash line in those figures. A carrier frequency is issupplied by oscillator at station A. The synchronous switchingmechanisms 310 and H0 at stations A and B respectively, which connectwith the poly-phase power systems 5l0 and H0, respectively, eachtransmit the same frequency h, at the two stations and with thecompensated pilot linemay be similar to the arrangement shown in Fig.11. At station B phase angle meters are shown for each phase of thepower circuit 200A, 200B and 200C for phases A, B and C, respectively ofthe power systems. By means of the synchronous switching arrangement therelative phase difiference between the corresponding phases of the twosystems may be successively measured on the respective phase anglemeters. One phase angle meter might be used by positioning it in thecommon circuit between the automatic phase compensating apparatus 299and. the synchronous switch 0.

Fig. 16 shows an arrangement employing car rier current forautomatically compensating a pilot line for phase change variationsbetween two points in which three difierent frequency carrier currentsmodulated by the power currents of the difierent respective phases ofthe power system are transmitted over the automatically compensatedpilot line for simultaneously making phase comparisons of the powercurrents or voltages. Each phase of the power circuit 5! employsindividual automatic phase compensating-equipment 99A, 99B and 990,respectively. This compensating equipment may be similar to that shownwithin the dash-dot line at the station A of Fig. 13 or 14. Carriercurrents of different frequencies in, fs and ii are supplied by sources90A, 90B and 900, respectively, for each of the different phases of thepower system having a frequency f1. Connection is made with the pilotline I00 through suitable filters 03A, 03B and 93C for each of the threephases, respectively. If the automatic compensating arrangement issimilar to that shown in Fig. 13, these filters transmit frequencies(f1+f2), (fl-H3) and (fr-H4), respectively. While if the automatic phasecompensating equipment is similar to that shown in Fig. 14, thesefilters transmit frequencies (fr-H2), and 2(i1-l-f2), (f1+fs) and2(f1+f3), and (fl-H4) and 2(f1+f4), respectively. At station B theautomatic phase compensating equipment 299A, 299B and 299C for therespective phases of the power system may be similar to that shown inFig. 13 or 14. Respective filters 293A, 2933 and 293C connect theautomatic phase compensating equipment of the different phases with thecompensated pilot line I00. Phase angle meters 200A, 2003 and 200C inthe respective aseaoss ment instead of successively as shown in Fig. 15v where synchronous switches are used at stations A and B. V

Fig. 17 shows an arrangement employing carriercurrent for automaticallycompensating a f are transmitted over the respective phases oftheautomatically compensated three-phase pilot line for simultaneouslymaking phase comparisons of the power currents or voltages. In thisarrangement the three-phase compensated pilot line is employed between{stations A and" B and each phase of this pilot lineis compensated"simultaneously and continuously. Separate automatic phase'conipen'satingunits-99A, 99B and 990 are used'--- with each phase-of the pilot lineI05 at' The carrier fre- V quencies for each phase maybe the same it ispreferabletojusethree diflerentfrequencies f2, faan'd f4 for the threedifferent phases respectively. Thefthree phases of a power line 5H1 areproperly connected to the terminal apparatus at station A and the threephases of a power line 6H] are properly connected to the terminalapparatus at station B. This frequency of the current on the powersystems is f1. At station B phase angle meters 200A, 2003 and 200Ccontinually show the phase relationship of the currents or voltages inthe two power systems 5H) and 6H] for the respective phases of the twosystems. In the drawing at station A three different oscillators havebeen shown each providing a different frequency carrier current.However, one oscillator providing a carrier current of the samefrequency for all threephases of the compensating equipment might beused as stated above but in this case each circuit of the pilot lineshould be better balanced than in the case where three differentfrequencies are used.

Fig. 18 shows an arrangement employing carrier current for automaticallycompensating one phase of ,a three-phase power line for phase changevariations between two points in which The terminal equipment in thisarrangement is quite similar to that of Fig. 13 except that one phase ofthe power line is employed for the pilot transmitting medium instead ofa separate pilot line. In either arrangement the pilot line circuit isused for other purposes, namely, in Fig. 13 it is used for telephony orother signaling, and in Fig. 18 it is also the circuit of one phase ofthe I power system. Two of the wires or one phase of the three-phasepower line I05 are used as the pilot circuit. Coupling condensersconnect the two wires used as the pilot line to the terminal apparatusat stations A and B. The condensers Qmust be capable of withstanding thepower line voltages but at the same time provide a high impedance to thepower frequency and a low impedance to the carrier frequency, asheretofore stated. In addition to prevent reflection and to isolate theapparatus from the effects of switching on the power circuit, it isdesirable to include at each end of the pilot line chokes MB and Hill,and 206 and 201, respectively, in series with the power line wires. Eachchoke may be tuned with a capacitance to make it anti-resonant to thepilot frequency. The automatic phase compensating apparatus at stationsA and B may be similar to that shown in- Fig. 13 and is here so shown.Alternating, current of frequency f1 from source In modulates inmodulator 9| a car.-

rier current of-frequency 1 from oscillator 0 at station A and the sum(or difference) of these currents is transmitted over the pilot line andreturned to cause the automatic compensation of the pilot line as.explained in connection with Fig. '13. Alternating current of frequencyf1 from source 2H) atstation B is impressed upon one side of the phaseangle meter 20!] and a part of the current transmitted from station A tostation B is transmitted to demodulator 29l and in turn that having afrequency of ii to the other side of the phase'a'ngle meter 20!]. Thusthe phase relationshinof the currents or voltages generated at station"Alby' .sourc'e J0. and at station 28 by source Zillare comparedby thephase angle meter Fig. i9 shows an arrangement employing carrier currentforautomatically compensating one phase of a three-phasepower line forphase change variations between two points in which compensation isachieved by transmitting a modulated carrier current in one directionand returning'a harmonic frequency thereof over the compensated onephase of the power line wires and arrangements at one point for makingcomparisons of phase relationship of currents or voltages at the twodifferent points. In this arrangement the automatic phase compensatingequipment at station A and at station B is similar to that shown withinthe dot-dash boundarylines of Fig. 14 but arranged to employ two wiresof one phase of a poly-phase power line for the pilot line between thetwo stations. The automatic compensating equipment at station A and atstation B is connected to one phase of the power line I05 at eachstation by means of siiitable coupling condensers. These condensers mustWlthStfJd the power line voltages and at the same time provide a highimpedance to the power frequencies and! a low impedance to the carrierfrequencies. At each station choke coils I06, H31 and I08, and 206, 20'!and 208, respectively, are inserted in series in. the power line wiresto prevent refiectio' and to isolate the terminal apparatus from theeffects of switching on the power circuits. At stationA alternatingcurrent of frequency I1 from the source I0 modulates in modulator 9| acarrier current of frequency f2 from the oscillator 90. The sum (ordifierence) components are transmitted over one phase of the power lineI05 and a harmonic thereof is returned over the same pair of wires tothe automatic compensating equipment at ;tation A for compensating thepilot circuit as ieretofo're explained. At station B alternating :urrentof frequency h from source 2H3 is im- Jressed upon the phase angle meter200, while L part of the current transmitted from station A after beingdemodulated in demodulator 29B is mpressed with a frequency of f1 uponthe other ide of the phase angle meter 200. The phase .ngle relationshipof the alternating currents renerated by the two sources is shown by therhase angle meter.

Fig. shows an arrangement employing carier current for automaticallycompensating one base of a three-phase power line for phase hangevariations between two points in which ompensation is achieved bytransmitting a modlated carrier current in opposite directions over nophase of the power line between the two aints and in which synchronousswitching lBChanlSm is used to connect in one phase after aother of thepower circuits for making phase imparisons of the power currents orvoltages the three-phase system at the two diiferent )iIltS. Theautomatic phase compensating uippient at each station A and B may be of1y of the several types heretofore shown and illustrated in this drawingfigure it is similar that shown in Fig. 13. This terminal equipent isfinnected at each station through suit- |le coupling condensers to twowires or one iase of the power line I05 and operates to :tomaticallycompensate this one circuit of the wer line. Synchronous switchingmechanism 0 and H0 at stations A and B, respectively, arranged toconnect in succession correspondg phases of the power system at the twostans. At station A current from the phase of 9 power system which isconnected through a synchronous switch 3) modulates in moduvor 9| acarrier current of frequency )2 genited by oscillator 96 and carriercurrent comments are produced for transmission over the 0 wires of thepower system used as a pilot e to station E and a part returned foractuatthe automatic compensating apparatus as scribed more in detail inthe description of 13. The different phases of the power Sys- [1 orsystems are connected as heretofore exined through suitable couplingcondensers at :h station to the respective synchronous .tchingmechanism. This synchronous switchmechanism may be of any well-knowntype.

this arrangement using a compensated one.

ass of the power line for a pilot line, the eral phases of a power line0r two difierent ver systems transmitting currents having fre.. ency f1may have the phase relationships of ir currents or voltages compared inthe phase {is meter 200 at station B by successively nect ng one phaseafter another of the power as by means of the synchronous switches.

'ig. 21 shows an arrangement employing carcurrent 'for automaticallycompensating for three phases of a three-phase power line phase changevariations between two points which compensation is achieved bytransmit- 5 three different suitably modulated carrier rents in oppositedirections over the power wires of each phase between the two points andarrangements at one point for simultaneously making comparisons or phaserelationship of. the currents or voltages in the difierent phases of thepower circuits at the two different points. This arrangement is somewhatsimilar to that shown in Fig. 17 in which the pilot line is athree-phase signal circuit while in this case, the pilot line uses thecircuits of a three-phase power system superimposing the automatic phasecompensating on the power line at the same time the power line istransmitting its power currents. The automatic phase compensatingapparatus 99A, 99B and 990 at station A and 299A, 299B and 2990 atstation B are connected through suitable protective coupling condensersto the difierent phases, A, B and C of the power line I05, respectively.Suitable choke coils are inserted in the power line wires to preventreflection and to isolate the apparatus from the efi'ect of switching onthe power circuit as illustrated. The power line as here indicated isenergized at each end with alternating current having a frequency of hby suitable generators or input transformers, the details of which arenot shown. The phase angle relationships of the two sources of power arecontinually shown in this arrangement by the phase angle meters 200A,2003 and 2000 at station B. The three phases of the power line may beautomatically compensated by any of the arrangements heretofore shownthough the one illustrated is similar to that shown in Fig. 17. For eachphase the power line current modulates a carrier frequency furnished byoscillators A, 90B and QiiC of fz equencies f2; f3 and ft for phases A,B and C, respectively. Components of modulation for each phase aretransmitted over the power circuit and a part returned to thecompensating equipment at station A. thus causing continual compensationof the power line circuits, so that the phase angle meters at station Bcontinually show the phase relationship of the two sources of power ateach end or the power line regardless of variations in phase change ofthe power line itself.

Fig. 22 shows an arrangement employing carrier current for automaticallycompensating one phase of a three-phase power line for phase changevariations at three frequencies between two points in which compensationis achieved by transmitting three different modulated carrier currentfrequencies in opposite directions over the compensated one phase of thepower line wires between the two points and arrangements at one pointfor simultaneously making comparisons of the phase relationship of thecurrents or voltages. of the difl'erent phases of the threephase powercircuits at the two different points. In Fig, 21 an arrangement wasshown for making phase relationship comparisons simultaneously of allthree phases of a power system between two different points bycompensating all three phases of the power line circuits and also usingthem as pilot line circuits. In Fig. 19 an arrangement is shown forusing two wires or one phase of a three-phase power line'as thecompensated line for making phase relationship comparisons of currentsor voltages at two difierent points on all three phases of the powerline. Fig. 22 compensates two wires or one circuit of the power linebetween two points at three different frequencies and provides forsimultaneously making the phase relationship comparisons on all threephases of a power system'or systems for two different points. The phasecompensating. equip-

